In general, an electric heater is a heating apparatus which uses heat generated when a current is conducted through a heating element. Electric heaters are typically manufactured by winding or processing heating wires in a specific shape. For instance, the electric heaters may have a structure, in which heating wires wound onto a mica plate or wound in a certain shape are placed inside a quartz tube, or in which a specific material is filled between heating wires and a stainless tube having a certain distance therebetween after the heating wires are placed in the stainless tube.
The related art electric heater, i.e., the heater using the heating wires, may have a problem of oxidation caused by direct contacting by air when heated as well as a structural problem to support the heating wires.
Considering these problems, a so-called plane heater which is formed by coating a heating element onto a substrate of a ceramic, glass, aluminum, stainless steel, etc. has recently been used. Such a plane heater facilitates a space utilization due to its thinness, and is good for an instantaneous heating due to its short heat transfer path, thereby being increasingly used.
FIG. 1 is an exemplary perspective view of a related art electric heater, and FIG. 2 is an enlarged view showing the main parts of the electric heater in FIG. 1. As shown in the drawings, an electric heater 20 may include a plate-shaped base 21 installed to be exposed on an upper surface of a table or a kitchen counter 10, or the like, a heating unit 31 formed on a surface of the base 21, and a power connection unit 41 for supplying power to the heating unit 31.
The base 21 is made of a glass member and is formed to have a plate shape. Generally, cooking utensils (not shown) are placed on the surface of the base 21, i.e., on an upper surface of the base 21. The heating unit 31 and the power connection unit 41 are disposed at another surface of the base 21, i.e., at a lower surface of the base 21. Indication lines 22 for indicating the positions where the cooking utensils are to be placed and a plurality of manipulation buttons 24 for manipulating the heating unit 31, are provided on the upper surface of the base 21.
Further, the heating unit 31 may include an electric resistor implemented as arc-shaped portions 32 and concentrically spaced from each other in a radial direction from the central portion, and curved portions 33 for connecting the arc-shaped portions 32, thereby forming a consecutively connected curved line shape. The heating unit 31 is connected to the power connection unit 41 so as to be connected to the mains power.
However, in the related art electric heater, when the heating unit 31 is formed in the consecutively curved line shape, a plurality of curved portions 33 having a relatively small curvature (radius of curvature) are formed, which easily leads to a thermal stress concentration therein. Accordingly, cracking, dielectric breakdown, etc. may easily occur at the curved portions 33 or at a periphery of the curved portions 33, compared to at the arc-shaped portions 31 or at a periphery of the arc-shaped portions 31, thus to cause a problem of reducing its service life.
FIG. 3 is an exemplary bottom view showing another related art electric heater. As shown in FIG. 3, an electric heater 50 may include a base 51 disposed to be exposed on an upper surface of a kitchen counter, a table, etc., and a heating unit 61 disposed on a plate surface of the base 51.
The heating unit 61 configured to heat a cooking utensil is formed at another surface of the base 51, i.e., at a lower surface thereof. The heating unit 61 may include arc-shaped portions 63 having an arc shape and concentrically disposed with respect to each other, and connecting portions 65 connecting the arc-shaped portions 63. The heating unit 61 is connected to a power connection unit 71 so as to be connected to the mains power. The heating unit 61, i.e., the arc-shaped portions 63 and the connection portions 65 are implemented as an electric resistor having a large electrical resistance value.
Meanwhile, the cooking utensil may be placed eccentrically, without being aligned onto the center of the heating unit 61. As shown in the dashed line 67 in FIG. 3, if the cooking utensil is placed eccentrically, the heating unit 61 may be divided into two areas comprised of a contact area 68 contacted by the cooking utensil and capable of heat conduction to the cooking utensil, and a non-contact area 69 not contacted by the cooking utensil and thusly incapable of heat conduction to the cooking utensil. Here, the interface formed between the contact area 68 and the non-contact area 69 increases the probability of a thermal/electrical breakdown due to an increase in the temperature gradient.
In the related art electric heater, however, the heating unit 61 is concentrically disposed in a long-and-thin arc shape. Thus, the interface between the contact area 68 and the non-contact area 69 becomes long, thereby increasing the probability of a thermal/electrical breakdown. Accordingly, damage or cracking easily occurs, thus to reduce its life.
In addition, the connection portions 65 connecting the arc-shaped portions 63 are structured to have a relatively small radius of curvature, thereby causing a concentration of the thermal stress (or temperature stress), thus to generate cracking or a dielectric breakdown even at a relatively low power level. Accordingly, there is a limit to enhance the output by more than a certain level.